Currently, a via on a substrate plays a role of providing a vertical path between interconnections. Under a low frequency operation, the paths formed by the vias are not very necessary to control the impedance thereof since the dimension of the via is really quite small (for example, <0.1λ) relatively to the wavelength (λ) of low operation frequency. At the time, the vias can be respectively treated as a simple electrical connection point, the length of the via can be neglected and it affects the circuit inconsiderably.
When the operation frequency is increased however, the physical dimension of a via is near to the wavelength of the operation frequency (for example, 1λ. At the time, the via should be treated as an additional component for a circuit. Hence, the additional via component in the circuit needs to be designed and under control so that the circuit can normally function.
A traditional via connects one terminal only, so that a return path must be designed to accomplish the signal transmission. As a result, during the traditional via is fulfilling a signal transmission, at least two vias are often needed, wherein one is signal via and another is ground via and/or power via. The second via is for providing a return path and solving the problem related to signal transmission integrity, as shown in FIGS. 1A and 1B.
In FIG. 1A, for example, a printed circuit board (PCB) with four layers includes two up and down signal layers 120 and 150 and a ground layer 130 and a power layer 140 both located between the two layers 120 and 150. A signal via 110 is disposed between the up signal layer 120 and the down signal layer 150. In FIG. 1A, a current path 102 from a source to a load and a current return path 104 are shown. In order to improve the signal transmission integrity, at least two vias are often designed as shown by FIG. 1B, in which one is signal via 110 and another is ground via and/or power via. In the example, the return path is implemented by a ground via 160. With the above-mentioned design, the length of the return path can be reduced and the signal integrity can be improved. In fact, the return path needs to be figured out with an effective design so as to achieve the design requirement of impedance match and reducing EMI.